Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes

Abstract

We present a transformative route to obtain mass-producible helical slow-wave structures for operation in beam−wave interaction devices at THz frequencies. The approach relies on guided self-assembly of conductive nanomembranes. Our work coordinates simulations of cold helices (i.e., helices with no electron beam) and hot helices (i.e., helices that interact with an electron beam). The theoretical study determines electromagnetic fields, current distributions, and beam− wave interaction in a parameter space that has not been explored before. These parameters include microscale diameter, pitch, tape width, and nanoscale surface finish. Parametric simulations show that beam−wave interaction devices based on selfassembled and electroplated helices will potentially provide gain-bandwidth products higher than 2 dBTHz at 1 THz. Informed by the simulation results, we fabricate prototype helices for operation as slow-wave structures at THz frequencies, using metal nanomembranes. Single and intertwined double helices, as well as helices with one or two chiralities, are obtained by selfassembly of stressed metal bilayers. The nanomembrane stiffness and built-in stress control the diameter of the helices. The inplane geometry of the nanomembrane determines the pitch, the chirality, and the formation of single vs intertwined double helices.

Formation and Shape Changing of Conductive Helical Ribbons via Deposition of Highly Stressed Films on Mechanically Responsive Substrates

“…Metal helices obtained by self-assembly of tethered ribbons have potential applications as on-chip transformers [1,2] and inductors, [3,4] microscale actuators, [5] metamaterials, [6] and millimeter-through THz slow-wave structures [7][8][9] for vacuum electronic devices. The diameter and pitch of helical structures required for these diverse applications range from several tens DOI: 10.1002/adfm.202312333 of nanometers to hundreds of micrometers, with the greatest opportunities appearing for robust helices with a micrometer-scale cross-section and pitch.…”

“…The most promising approaches for scalable fabrication of conductive helices rely on the deposition, release, and self-assembly of stressed metal layers patterned into ribbons. [7] The equilibrium diameter of the helical ribbons is determined by the balance of a net bending moment, proportional to the residual stress gradient in the metal heterostructure and internal resistance forces, which are more significant in stiffer metals. [10][11][12] Inherently large stresses in metals would require depositing 0.5-10 μm-thick films to obtain helices with 0-500 μm diameter via self-assembly of stressed metal ribbons.…”

“…[27][28][29][30] Continuum mechanics modeling also demonstrates that tailoring stress profiles during electrodeposition provides an effective approach to fabricating 3D metal structures with a high degree of control over their geometry. [7]…”

Slow waves on long helices

“…Recent fabrication improvements have also meant helical structures operating at higher frequencies have become possible, e.g. at terahertz frequencies 38 . However, while it has been shown that the frequency bandwidth of slow waves in helical waveguides and antennas is limited 20 , 34 , 35 , there has been no systematic study attempting to characterise, understand and improve upon bandwidth limitations.…”

Study of an Attenuator Supporting Meander-Line Slow Wave Structure for Ka-Band TWT

“…As a result, the planar slow wave structure (SWS) has attracted wide attention over the last decade. One kind of self-winding helix quasi-planar SWS is explored and fabricated by using MEMS technology for potential mass production [10]. The planar SWS features as almost two-dimensional rather than three-dimensional.…”